Cu-Catalyzed Dehydrogenative C-O Cyclization for the Synthesis of Furan-Fused Thienoacenes

Article abstract of DOI:10.1021/acs.orglett.1c01256

The first Cu-catalyzed dehydrogenative C-O cyclization for the synthesis of furan-fused thienoacenes is described. A variety of heteroacenes including a thieno[3,2-b]furan or a thieno[2,3-b]furan skeleton were synthesized by intramolecular C-H/O-H coupling. The use of a mixed solvent of N-methyl-2-pyrrolidone, ethylene glycol monomethyl ether, and toluene was essential for suppressing side reactions and efficiently promoting the reaction. Double C-O cyclization was also conducted to afford highly π-expanded furan-fused thienoacenes.

Full text of DOI:10.1021/acs.orglett.1c01256

pubs.acs.org/OrgLett
Letter
Cu-Catalyzed Dehydrogenative C−O Cyclization for the Synthesis of
Furan-Fused Thienoacenes
Koichi Mitsudo,* Yoshiaki Kobashi, Kaito Nakata, Yuji Kurimoto, Eisuke Sato, Hiroki Mandai,
and Seiji Suga*
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ABSTRACT: The first Cu-catalyzed dehydrogenative C−O cyclization for the
synthesis of furan-fused thienoacenes is described. A variety of heteroacenes
including a thieno[3,2-b]furan or a thieno[2,3-b]furan skeleton were
synthesized by intramolecular C−H/O−H coupling. The use of a mixed
solvent of N-methyl-2-pyrrolidone, ethylene glycol monomethyl ether, and
toluene was essential for suppressing side reactions and efficiently promoting
the reaction. Double C−O cyclization was also conducted to afford highly π-
expanded furan-fused thienoacenes.
he formation of carbon−oxygen bonds plays a significant
role in organic synthesis because C−O bonds are an
Scheme 1. Representative Transition-Metal-Catalyzed
Dehydrogenative C−O Coupling Reactions
T
abundant and ubiquitous moiety in nature,1 pharmaceuticals,2
and organic materials.3 A modern method for constructing C−
O bonds is the Buchwald−Hartwig etherification reaction.4
Recently, transition-metal-catalyzed dehydrogenative coupling
reactions have also received considerable attention as a novel
and straightforward method for constructing carbon−heter-
oatom bonds with high atom economy.5 Whereas there have
been many reports on dehydrogenative C−N couplings,6 there
have been few reports on the construction of C−O bonds by
dehydrogenative coupling.7,8
In 2010, Yu and coworkers reported the first Pd-catalyzed
dehydrogenative C−O cyclization to form dihydrobenzofurans
under oxidative conditions (Scheme 1a).7a The following year,
Liu7b and Yoshikai7c independently reported the Pd-catalyzed
dehydrogenative synthesis of dibenzofuran derivatives
(Scheme 1b). Zhu and coworkers reported that copper
catalysts could also be used for dehydrogenative etherifications
(Scheme 1c).7d In 2013, Yu,8a Wang,8b Martin,8c and
Gevorgyan8d reported the C−O cyclization of carboxylic
acids to give lactones. To the best of our knowledge, the scope
of these reported reactions has been limited to aryl ethers and
lactones and there have been no reports on the construction of
heteroaromatic ethers.
Meanwhile, heteroacenes, acenes involving heteroaromatic
rings, have attracted attention as functional organic materials
with excellent atmospheric stability and solubility. In particular,
heteroacenes including thiophene,9 furan,10 or both11 have
been recognized for their properties as organic field effect
transistors (OFETs). We have been interested in the
development of novel synthetic methods for heteroacenes.12
Our attention has recently been focused on dehydrogenative
C−O cyclization for the construction of furan-fused
thienoacenes. In our preliminary study, Pd-catalyzed dehydro-
Received: April 13, 2021
Published: May 24, 2021
https://doi.org/10.1021/acs.orglett.1c01256
© 2021 American Chemical Society
Org. Lett. 2021, 23, 4322−4326
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Letter
genative C−O cyclization was examined under several
conditions, but the desired reaction did not proceed efficiently.
We next turned our attention to Cu-catalyzed reactions and
found that the designed reaction proceeded under the
appropriate conditions (Scheme 1d). Heteroacenes including
a thieno[3,2-b]furan or thieno[2,3-b]furan skeleton could be
fabricated by this method. We report here the first copper-
catalyzed intramolecular dehydrogenative C−O bond for-
mation for the synthesis of furan-fused thienoacenes.
Whereas several reaction conditions were examined, the
efficiency of the reaction was still insufficient. One problem is
that complex mixtures were also obtained with 2a, and thus the
suppression of side reactions should significantly promote the
reaction progress. To overcome this situation, we tried several
ligands, additives, and higher and lower reaction temperatures,
but all of these attempts were unsuccessful.13 Finally, we
optimized the solvents for Cu-catalyzed dehydrogenative
cyclization using 0.1 M of 1a (Table 2). Other polar aprotic
First, we chose 2-(benzo[b]thiophen-2-yl)phenol (1a) as a
model compound, and Cu-catalyzed dehydrogenative C−O
cyclization was examined under several conditions (Table 1).
Table 2. Effect of Solvents for Cu-Catalyzed
Dehydrogenative C−O Cyclization of 1a
a
Table 1. Cu-Catalyzed Dehydrogenative Cyclization of 1a
a
under Several Conditions
b
entry
solvent
1a (%)
2a (%)
c
1
2
3
4
5
6
7
8
9
NMP
DMF
DMSO
EGM
EGM
NMP/EGM (1:1)
NMP/EGM (1:1)
NMP/EGM/toluene (1:1:2)
NMP/EGM/toluene (1:1:2)
N.D.
59
18
6
31
50
69
74
N.D.
N.D.
60
trace
N.D.
N.D.
N.D.
N.D.
b
entry
[Cu]
CuBr
CuI
base
acid
yield (%)
c
1
2
3
4
5
6
7
8
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
K2CO3
K3PO4
KO(t-Bu)
KOAc
KOAc
KOAc
KOAc
NaOAc
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
30 (27)
18
6
38
41
44
43
46
48
19
d
CuOTf
e
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
ef
,
g
78 (73)
efg
, ,
g
62
a
Reaction conditions: 1a (0.20 mmol), Cu(OAc)2 (30 mol %),
NaOAc (20 mol %), PhCOOH (50 mol %), solvent (0.1 M), 145 °C,
b
1
9
AcOH
air, 3 h. Determined by H NMR with 1,1,2,2-tetrachloroethane as
c d e
an internal standard. Not detected. Performed for 6 h. Performed
10
11
12
CF3COOH
PhCOOH
PhCOOH
f
g
h
d
with 0.05 M of 1a. Performed for 24 h. Isolated yield. Performed
52 (55)
de
,
on a 1.0 mmol scale.
59
a
Reaction conditions: 1a (0.20 mmol), [Cu] (30 mol %), base (20
mol %), acid (50 mol %), NMP (0.2 M), 145 °C, air, 24 h.
solvents such as DMF and DMSO were ineffective (entries 2
and 3). In ethylene glycol monomethyl ether (EGM), 2a was
obtained in 31% yield. Interestingly, the decomposition of 1a
was almost suppressed in EGM, and 60% of 1a was recovered
(entry 4). When the reaction time was extended to 6 h, 1a was
mostly consumed, and the yield of 2a increased to 50% (entry
5). We considered that EGM would work as a ligand to
suppress the side reaction, but its polarity is not enough to
efficiently promote the C−O cyclization. To overcome this
situation, we next examined the reactions in mixed solvents
(entries 6−8). When the reaction was carried out in a mixture
of NMP and EGM (1:1), the yield of 2a increased to 69%
(entry 6). We next conducted the reaction with 0.05 M of 1a,
and 2a was obtained in 74% yield (entry 7). Further tuning
revealed that a mixture of NMP, EGM, and toluene (1:1:2)
was the best solvent for the reaction (entry 8). Whereas a
longer reaction time was required (24 h), 2a was obtained in
the highest yield (78% NMR yield, 73% isolated yield). With a
1.0 mmol scale, 2a was obtained in 62% yield (entry 9).
To clarify the scope of the reaction, we next examined the
Cu-catalyzed dehydrogenative cyclization of several 2-(benzo-
[b]thiophen-2-yl)phenol derivatives 1 under the optimized
conditions (Scheme 2). First, p-substituted 2-(benzo[b]-
thiophen-2-yl)phenols 1b−h were subjected to the reactions.
Cu-catalyzed dehydrogenative C−O coupling proceeded
smoothly, and the corresponding eight-substituted benzo[4,5]-
thieno[3,2-b]benzofurans 2b−h were obtained, although the
yields of the products were low in some cases (2c and 2e). A
b
Determined by 1H NMR with 1,1,2,2-tetrachloroethane as an
c
internal standard. Performed with base (50 mol %) and acid (1
d
e
equiv) in DMSO. Performed with 0.1 M of 1a. Performed for 3 h.
First, we carried out the cyclization under conditions similar to
those reported by Zhu.7d In the presence of CuBr (30 mol %),
Cs2CO3 (50 mol %), and t-BuCO2H (PivOH; 1 equiv), 1a in
DMSO was heated to 145 °C under air. Although 1a was
completely consumed, product 2a was obtained in only 27%
yield (in parentheses in entry 1). After several attempts at
optimization, we found that N-methyl-2-pyrrolidone (NMP)
was a better solvent than DMSO, and 20 mol % of Cs2CO3 and
50 mol % of PivOH were enough to promote the reaction
(30% yield, entry 1). We next optimized the copper source
(entries 2−4). Among the Cu catalysts we have tried so far,
Cu(OAc)2 gave the best results, and the yield of 2a increased
to 38% (entry 4). We next optimized the acids and bases
(entries 5−12). The addition of a base and an acid was
essential for the reaction and influenced the reaction efficiency.
The use of KOAc instead of Cs2CO3 slightly increased the
yield of 2a (46%, entry 8). The use of PhCO2H instead of
PivOH also slightly increased the yield of 2a (52%, entry 11).
The concentration of the reaction solution positively affected
the reaction. When the solution was diluted to 0.1 M, the yield
of 1a increased to 55%. With NaOAc instead of KOAc, the
reaction was finished within 3 h to give 2a in 59% yield (entry
12).
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Scheme 2. Cu-Catalyzed Dehydrogenative C−O Cyclization
Scheme 3. Cu-Catalyzed Dehydrogenative C−O Cyclization
a
a
of 1 under the Optimized Conditions
to Form Heteroacenes Involving Thieno[2,3-b]furan
a
Reaction conditions: 1 (0.20 mmol), Cu(OAc)2 (30 mol %), NaOAc
(20 mol %), PhCOOH (50 mol %), NMP/EGM/toluene (1:1:2, 0.05
b
M), 145 °C, air, 24 h. Isolated yield. Performed at 135 °C.
c
Performed from 2-(2-hydroxyphenyl)benzo[b]thiophene 1,1-dioxide
(1p).
a
Reaction conditions: 3 (0.20 mmol), Cu(OAc)2 (30 mol %), NaOAc
variety of substrates with electron-donating or -withdrawing
groups could be applied for the reaction. We next used o- or
m-substituted 2-(benzo[b]thiophen-2-yl)phenols 1i−k, and
the corresponding substituted 2-(benzo[b]thiophen-2-yl)-
phenols 2i−k were obtained. From substrate 1l bearing a 2-
naphthol, further π-expanded heteroacene 2l was obtained in
good yield (86% yield). From 1m or 1n with substituted
benzo[b]thiophene moieties, the corresponding substituted
products 2m and 2n were obtained. Whereas a substrate
having a thiophene unit (1o) could also be used in the
reaction, the yield was insufficient. Interestingly, C−O
cyclization also proceeded from 1p having a benzo[b]-
thiophene 1,1-dioxide to afford 2p.
Under the optimized conditions, substrates having 3-thienyl
groups 3 were also applicable (Scheme 3). From 2-(benzo-
[b]thiophen-3-yl)phenol (3a), the desired heteroacene 4a
including the thieno[2,3-b]furan skeleton was obtained in an
excellent yield (90%). Similarly, several mono- and disub-
stituted heteroacenes 4b−g were obtained from the corre-
sponding substrates 3b−g. A bromo group, which can be easily
transformed to other groups, was tolerated under the reaction
conditions (3e−g). With 2-(thiophen-3-yl)phenol (3h), C−O
cyclization proceeded without a base to give the product 4h in
good yield (83%). From 3i, a substrate bearing a naphthyl
group, naphtho[2,1-b]thieno[3,2-d]furan (4i), was obtained in
excellent yield (95%). Notably, C−O cyclization of substrates
having two hydroxy groups also proceeded smoothly to afford
highly π-expanded heteroacenes such as 4j and 4k in good
yields (76 and 89% yield, respectively). The structures of 4j
and 4k were confirmed by X-ray crystallography. Whereas the
(20 mol %), PhCOOH (50 mol %), NMP/EGM/toluene (1:1:2, 0.05
b
M), 145 °C, air, 24 h. Isolated yield. Performed without NaOAc.
c
Performed with Cu(OAc)2 (0.6 equiv), NaOAc (0.8 equiv), and
PhCOOH (2 equiv) in NMP/EGM/toluene (1:1:2, 0.0125 M) under
d
O2. Performed for 21 h.
reactions proceeded in air, the use of oxygen gave better results
(Table S9).
To obtain further insight into the reaction, control
experiments were carried out (Scheme 4). We first carried
out the Cu-catalyzed cyclization of 1q-d (Scheme 4a) to
investigate the kinetic isotope effect (KIE) and found that D%
of the recovered 1q-d was unexpectedly low (14%D). To
clarify the reason for this observation, another D-labeled
experiment was conducted with D-labeled 1l-d (Scheme 4b).
After heating for 6 h, cyclized product 2l was obtained in 44%
yield, and 47% of 1l-d was recovered (49%D). The drastic
decrease in D% of 1l-d strongly suggests that cleavage of the
C−H bond would be a reversible process.14 Finally, the
stability of the product 2l was confirmed under the reaction
conditions (Scheme 4c). After the treatment of 2l for 24 h
under the reaction conditions, 95% of 2l was recovered,
meaning that 2l was stable under these conditions. We next
carried out the Cu-catalyzed reaction of 1a in the presence of
TEMPO (Scheme S1). The yield of 2a decreased to 38%,
suggesting that the reaction would not proceed via a radical
pathway.
On the basis of the mechanistic studies, a plausible
mechanism is illustrated in Figure 1. First, Cu species bearing
carboxylates (R = Me or Ph) would react with 1 to form
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Scheme 4. Control Experiments
ASSOCIATED CONTENT
* Supporting Information
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The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c01256.
Experimental details and spectral data for all new
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Accession Codes
CCDC 2040856−2040857 contain the supplementary crys-
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AUTHOR INFORMATION
Corresponding Authors
■
Koichi Mitsudo − Division of Applied Chemistry, Graduate
School of Natural Science and Technology, Okayama
University, Okayama 700-8530, Japan; orcid.org/0000-
0002-6744-7136; Email: mitsudo@okayama-u.ac.jp
Seiji Suga − Division of Applied Chemistry, Graduate School
of Natural Science and Technology, Okayama University,
Okayama 700-8530, Japan; orcid.org/0000-0003-0635-
2077; Email: suga@cc.okayama-u.ac.jp
Authors
Yoshiaki Kobashi − Division of Applied Chemistry, Graduate
School of Natural Science and Technology, Okayama
University, Okayama 700-8530, Japan
Kaito Nakata − Division of Applied Chemistry, Graduate
School of Natural Science and Technology, Okayama
University, Okayama 700-8530, Japan
Yuji Kurimoto − Division of Applied Chemistry, Graduate
School of Natural Science and Technology, Okayama
University, Okayama 700-8530, Japan
Eisuke Sato − Division of Applied Chemistry, Graduate School
of Natural Science and Technology, Okayama University,
Okayama 700-8530, Japan; orcid.org/0000-0001-6784-
138X
Hiroki Mandai − Department of Pharmacy, Faculty of
Pharmacy, Gifu University of Medical Science, Gifu 509-
0293, Japan; orcid.org/0000-0001-9121-3850
Figure 1. Plausible mechanism for the dehydrogenative C−O
cyclization of 1.
intermediate A, which would be converted to metallacycle C
via transition-state B by a concerted metalation deprotonation
(CMD) process. The transformations between 1 and C should
be reversible processes, which is strongly suggested by the D-
labeled experiments. Finally, subsequent reductive elimination
would proceed irreversibly to afford product 2 with a low
valent copper species, which would be oxidized by oxygen to
regenerate active copper species. NaOAc would promote the
regeneration of the active copper species. Whereas the role of
EGM is not yet clear, we assumed that EGM would coordinate
with the copper species and prevent its decomposition.
In conclusion, we developed Cu-catalyzed dehydrogenative
C−O cyclization to afford furan-fused thienoacenes. Thienoa-
cenes including either thieno[3,2-b]furan or thieno[2,3-b]furan
skeletons were obtained under similar conditions. Double C−
O cyclization enabled easy access to highly π-expanded furan-
fused thienoacenes. The physical properties of the thus-
obtained heteroacenes and further application of this strategy
are under investigation in our laboratory.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c01256
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported in part by JSPS KAKENHI grant
numbers JP19K05477, JP19K05478, and JP18H04455 for
Middle Molecular Strategy and by JST, ACT-C, Japan.
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(13) For details of the screenings, see the Supporting Information.
(14) This result was highly different from that of Zhu and coworkers,
who observed a large KIE in the Cu-catalyzed dehydrogenative
cyclization for synthesizing dibenzorfurans; see ref 7d.
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